Sergio sent me a scenery package with the question: why don’t these two textures appear as the same color in X-Plane (they did in Photoshop). The answer is: gamma correction.
Some background (with much hand-waving): gamma refers to how bright the mid-tones of an image appear. (It’s a lot more complex than that, but that’s what Wikipedia is for!) Basically Macs and some other computers adjust the colors in an image to compensate for the deficiencies of CRTs, while PCs leave them alone. The result is that the same numeric color levels, when sent to Mac and PC hardware, result in brighter images on the Mac than the PC.
Since X-Plane is authored almost entirely on the Mac, the old complaint (around the time of X-Plane 6, with BMPs) was that X-Plane looked too dark on Windows. PNG addresses this issue: the gamma curve of the system an image was created on can be written into the PNG, allowing X-Plane to adjust the colors (making them brighter or darker) depending on what destination system we are running on.
Unfortunately, X-Plane isn’t too brilliant about this, in two ways, one of which isn’t our fault:
- X-Plane assumes the platform default gamma for both Mac and PC (that would be 1.8 for Mac and 2.2 for PC). If you read that background article, you know that this isn’t real clever of us. But it actually is better than doing nothing at all.
- Gamma in PNG files is optional; if we get a PNG file with no gamma information, we make the rather arbitrary assumption that it came from a Mac. Since X-Plane used to be authored on Macs, this seemed like a reasonable thing and in the case of no gamma information, we’re going to be wrong half of the time no matter what.
If you open up the default scenery PNG files in Preview, you can see the gamma inforation – 0.4545. This is 1/2.2, meaning the files are encoded to PC gamma standards. It turns out that one of the textures Sergio sent me had no gamma information, so X-Plane assumed Mac gamma (0.5555, or 1/1.8). Thus the brightness of these textures were being adjusted by different amounts.
My recommendation to authors is simple: make sure that all of your PNG files always have gamma values written into them. Otherwise there is a risk that the default gamma guess that X-Plane makes will not be the one you authored under, causing color shifts.